Lithium-ion battery and method of manufacturing the same

ABSTRACT

The lithium-ion battery includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode contains a negative electrode active material. The negative electrode active material contains a silicon material. The silicon material contains a silicon alloy phase and a silicate phase. The silicon alloy phase has a three-dimensional network structure. The silicate phase is arranged in a mesh of the three-dimensional network structure. The three-dimensional network structure has an average mesh size of 2.8 nm to 3.5 nm.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No.2019-234478 filed on Dec. 25, 2019, incorporated herein by reference inits entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a lithium-ion battery and a method ofmanufacturing the same.

2. Description of Related Art

Japanese Unexamined Patent Application Publication No. 2017-147247 (JP2017-147247 A) discloses that a negative electrode structure containingsilicon oxide is charged and discharged at a current rate of 1.1 C to3.0 C.

SUMMARY

A silicon (Si) material has been investigated as negative electrodeactive material of lithium-ion battery (hereinafter, may be abbreviatedas “battery”). The advantage of Si material is that a specific capacityis large. The disadvantage of Si material is that a cycle life is short.

Conventionally, various studies have been made in order to improve thecycle life of Si material. For example, in JP 2017-147247 A, solely anegative electrode containing a Si material is charged and discharged ata predetermined current rate before assembling a battery. JP 2017-147247A discloses that a Si network develops in a three-dimensional network inthe Si material by the charge and discharge. JP 2017-147247 A disclosesthat the cycle life is improved by forming the Si network. However, abattery containing a Si material has room for improvement in storagecharacteristics.

The present disclosure provides a battery containing a Si material withimproved storage characteristics.

Hereinafter, technical configurations and effects of the presentdisclosure will be described. Note that, the mechanism of action of thepresent disclosure includes an assumption. Whether the mechanism ofaction is right or wrong does not limit the scope of the claims.

A first aspect of the present disclosure relates to a lithium-ionbattery that includes a positive electrode, a negative electrode, and anelectrolyte. The negative electrode contains a negative electrode activematerial. The negative electrode active material contains a siliconmaterial. The silicon material contains a silicon alloy phase and asilicate phase. The silicon alloy phase has a three-dimensional networkstructure. The silicate phase is arranged in a mesh of thethree-dimensional network structure. The average mesh size of thethree-dimensional network structure is 2.8 nm to 3.5 nm.

The Si material according to the first aspect contains a Si alloy phaseand a silicate phase. The Si alloy phase contains an alloy of lithium(Li) and Si. The Si alloy phase has a three-dimensional networkstructure. It is considered that Li is mainly stored in the Si alloyphase. The silicate phase contains Li silicate. The silicate phase isarranged in a mesh of the three-dimensional network structure. With thestorage and release of Li, the Si alloy phase expands and contracts. Itis considered that the silicate phase may mitigate the volume change ofthe Si alloy phase. Furthermore, it is considered that the silicatephase may impede the decomposition reaction of the electrolyte.

The three-dimensional network structure of the Si alloy phase may beconfirmed in a Si distribution image by Scanning Transmission ElectronMicroscopy-Electron Energy-Loss Spectroscopy (STEM-EELS).

According to the first aspect, the denseness of the three-dimensionalnetwork structure may change depending on the condition of the initialcharging of the battery. Storage characteristics tend to be improvedwhen the three-dimensional network structure has an appropriatedenseness. That is, when the average mesh size of the three-dimensionalnetwork structure is 2.8 nm or more and 3.5 nm or less, the storagecharacteristics tend to be improved.

It is considered that the three-dimensional network structure (Si alloyphase) also functions as a Li transmission path. It is considered thatcapacity deterioration is less likely to occur when the Li transmissionpath has an appropriate denseness.

In the lithium-ion battery according to the first aspect, the negativeelectrode active material may further contain a carbon material.

A second aspect of the present disclosure relates to a manufacturingmethod of a lithium-ion battery. The manufacturing method includesassembling the lithium-ion battery and performing an initial charging onthe lithium-ion battery. The lithium-ion battery includes a positiveelectrode, a negative electrode, and an electrolyte. The negativeelectrode contains a negative electrode active material. The negativeelectrode active material contains a precursor of a silicon material.The precursor has a composition represented by SiO_(x). In the formula,the relationship of 0<x<2 is satisfied. The initial charging includes afirst step and a second step. In the first step, the charging isperformed to an intermediate voltage at a first current rate. In thesecond step, the charging is performed from the intermediate voltage toa maximum voltage at a second current rate. The first current rate islower than 0.5 C. The second current rate is higher than the firstcurrent rate. The intermediate voltage is 3.75 V or higher.

In the present disclosure, “C” is used as the unit of current rate. “1C” is defined as a current rate at which full charge capacity of thebattery is charged in one hour. For example, 0.5 C indicates a currentrate of 0.5 times 1 C. At the current rate of 0.5 C, the full chargecapacity is charged in two hours.

In the present disclosure, the initial charging is divided into twosteps. In the first step, the charging is performed to an intermediatevoltage at a relatively low current rate. In the second step, thecharging is performed from the intermediate voltage to the maximumvoltage at a relatively high current rate. Although the mechanism isunclear, under the conditions described in the second aspect, athree-dimensional network structure having an appropriate densenesstends to be formed.

In the manufacturing method of a lithium-ion battery according to thesecond aspect, the negative electrode active material may furthercontain a carbon material.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the disclosure will be described below withreference to the accompanying drawings, in which like signs denote likeelements, and wherein:

FIG. 1 is a first schematic view of a lithium-ion battery in the presentembodiment;

FIG. 2 is a second schematic view of the lithium-ion battery in thepresent embodiment;

FIG. 3 is a schematic cross-sectional view of a power storage element inthe present embodiment;

FIG. 4 is a first example of a Si distribution image by STEM-EELS;

FIG. 5 is a second example of the Si distribution image by STEM-EELS;and

FIG. 6 is a schematic flowchart of a manufacturing method of alithium-ion battery in the present embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present disclosure (hereinbelow, alsoreferred to as “the present embodiment”) will be described. Note that,the following description does not limit the scope of the claims.

In the present embodiment, for example, an expression such as “2.8 nm to3.5 nm” indicates a range including boundary values, unless otherwisespecified. That is, for example, “2.8 nm to 3.5 nm” indicates a range of“2.8 nm or more and 3.5 nm or less”.

Lithium-Ion Battery

In the present embodiment, “lithium-ion battery” means a secondarybattery that contains lithium (Li) ions as a charge carrier. The batteryin the present embodiment can be in any form. For example, the batterymay be in the form of a square battery, a cylindrical battery, or apouch-type battery. In the present embodiment, a pouch-type battery willbe described as an example. The pouch-type battery is also called a“laminate-type battery”.

FIG. 1 is a first schematic view of the lithium-ion battery in thepresent embodiment. A battery 100 is a pouch-type battery. The battery100 includes an exterior material 90. The exterior material 90 is apouch made of an aluminum laminated film. The exterior material 90 issealed. The exterior material 90 can be sealed, for example, by heatsealing. Each of a positive electrode terminal 81 and a negativeelectrode terminal 82 is exposed to the outside of the exterior material90.

FIG. 2 is a second schematic view of the lithium-ion battery in thepresent embodiment. The exterior material 90 accommodates a powerstorage element 50 and an electrolyte (not shown). That is, battery 100includes the power storage element 50 and the electrolyte. Each of thepositive electrode terminal 81 and the negative electrode terminal 82 isconnected to the power storage element 50.

FIG. 3 is a schematic cross-sectional view of a power storage element inthe present embodiment. The power storage element 50 is alaminate(stack)-type power storage element. The power storage element 50is formed by laminating three or more sheet-shaped electrodes. The powerstorage element may be a wound-type power storage element. That is, thepower storage element may be formed by spirally winding a belt-shapedelectrode.

The power storage element 50 includes a positive electrode 10, anegative electrode 20, and a separator 30. That is, the battery 100includes the positive electrode 10 and the negative electrode 20. Thepositive electrodes 10 and the negative electrodes 20 are alternatelylaminated. The separator 30 is arranged between the positive electrode10 and the negative electrode 20.

Negative Electrode

The negative electrode 20 has a sheet shape. The negative electrode 20may include, for example, a negative electrode current collector 21 anda negative electrode active material layer 22. The negative electrodecurrent collector 21 may have a thickness of, for example, 5 μm to 50μm. The negative electrode current collector 21 may contain a copper(Cu) foil or the like.

The negative electrode active material layer 22 is formed on a surfaceof the negative electrode current collector 21. The negative electrodeactive material layer 22 may be formed on solely one surface of thenegative electrode current collector 21. The negative electrode activematerial layer 22 may be formed on both front and back surfaces of thenegative electrode current collector 21. The negative electrode activematerial layer 22 may have a thickness of, for example, 10 μm to 200 μm.

The negative electrode active material layer 22 contains a negativeelectrode active material. That is, the negative electrode 20 contains anegative electrode active material. The negative electrode activematerial layer 22 may substantially consist of the negative electrodeactive material. The negative electrode active material contains a Simaterial. The negative electrode active material may substantiallyconsist of the Si material.

Si Material

The Si material may be a particle group (powder), for example. The Simaterial may have a median diameter of, for example, 0.01 μm to 20 μm.The Si material may have a median diameter of, for example, 0.1 μm to 10μm. The Si material may have a median diameter of, for example, 0.5 μmto 5 μm. The “Median diameter” in the present embodiment refers to aparticle diameter at which the cumulative particle volume from a smallparticle side in a volume-based particle diameter distribution accountsfor 50% of the total particle volume. The median diameter can bemeasured by a laser diffraction type particle diameter distributionmeasuring device or the like.

The Si material in the present embodiment is generated by the reactionbetween a precursor and Li at the time of initial charging. Theprecursor is an oxide of Si. The precursor has a composition representedby the following formula (I):

SiO_(x)  (I).

In the formula (I), the relationship of “0<x<2” is satisfied. Forexample, the relationship of “0.5≤x≤1.5” may be satisfied. For example,the relationship of “0.8≤x≤1.2” may be satisfied.

The Si material contains a Si alloy phase and a silicate phase. The Sialloy phase contains an alloy of Li and Si. The Si alloy phase maysubstantially consist of the LiSi alloy. It is considered that Li ismainly stored in the Si alloy phase. With the storage and release of Li,the Si alloy phase expands and contracts.

The silicate phase contains Li silicate. The silicate phase maysubstantially consist of the Li silicate. It is considered that thesilicate phase may mitigate the volume change of the Si alloy phase.Furthermore, it is considered that the silicate phase may impede thedecomposition reaction of the electrolyte.

The Li silicate may have a composition represented by, for example, thefollowing formula (II):

Li_(y)Si O_(z)  (II).

In the formula (II), for example, the relationship of “1≤y≤8, 2.5≤z≤6”may be satisfied. In the formula (II), the relationships of “y=z=4”,“y=2, z=3”, “y=1, z=2.5”, “y=3, z=3.5”, “y=2/3, z=7/3”, “y=8, z=6”, orthe like may be satisfied.

Average Mesh Size

FIG. 4 is a first example of a Si distribution image by STEM-EELS.

The Si alloy phase is three-dimensionally continuous. The Si alloy phaseforms a network skeleton. That is, the Si alloy phase has athree-dimensional network structure. In the Si alloy phase, metallic Siis distributed at a high concentration. The Si distribution image bySTEM-EELS is considered to represent the structure of the Si alloyphase. In FIG. 4, the white portion (bright portion) extending in athree-dimensional network is considered to represent the Si alloy phase.The black portion (dark portion) forms a mesh of the three-dimensionalnetwork white portion. The black portion is considered to represent thesilicate phase. That is, the silicate phase is arranged in the mesh ofthe three-dimensional network structure.

In the present embodiment, the average mesh size is 2.8 nm to 3.5 nm.Within the range, improvement in storage characteristics is expected.When the average mesh size is less than 2.8 nm, the desired storagecharacteristics may not be realized. Even though the average mesh sizeexceeds 3.5 nm, the desired storage characteristics may not be realized.The average mesh size may be, for example, 3.1 nm or more. The averagemesh size may be, for example, 3.3 nm or less.

Measuring Method of Average Mesh Size

The average mesh size in the present embodiment is measured by thefollowing procedure. The battery 100 is discharged to 2.5 V at a currentrate of 0.2 C. After the discharging, the negative electrode 20 iscollected by disassembling the battery 100. The negative electrode 20 iscleaned with a predetermined organic solvent. After the cleaning, asectional sample of the negative electrode active material layer 22 isproduced by cutting the negative electrode 20. The surface of thesectional sample is smoothed by Focused Ion Beam (FIB).

The sectional sample is observed by STEM. The observation magnificationis, for example, about 100,000 times to 500,000 times. An enlarged imageof the Si material is acquired as an Annular Dark Field-STEM (ADF-STEM)image. Further, the EELS spectrum is acquired by the EELS detector.

A STEM-EELS image is formed by imaging a position where the EELSspectrum of 15 eV to 18 eV is detected. That is, the Si distributionimage (for example, FIG. 4) is acquired. In the Si distribution image,the Si alloy phase is displayed as the white portion (bright portion).The silicate phase is displayed as the black portion (dark portion). Theunidirectional diameters of the black portion are measured at 20locations. In the present embodiment, the arithmetic mean value of theunidirectional diameters at 20 locations is regarded as the “averagemesh size”.

Carbon Material

The negative electrode active material may further contain a carbonmaterial in addition to the Si material. The Si material and the carbonmaterial may be compounded. Both large capacity and long cycle life areexpected to be achieved when the negative electrode active materialfurther contains a carbon material. The carbon material may be aparticle group, for example. The carbon material may have a mediandiameter of, for example, 1 μm to 20 μm. The carbon material may have amedian diameter of, for example, 1 μm to 10 μm.

The carbon material can contain any component as long as the carbonmaterial can function as the negative electrode active material. Forexample, the carbon material may contain at least one selected from thegroup consisting of graphite, soft carbon, and hard carbon.

In the present embodiment, the mixing ratio of the Si material and thecarbon material is arbitrary. For example, the relationships of “Simaterial/carbon material=1/99” to “Si material/carbon material=99/1” maybe satisfied. For example, the relationships of “Si material/carbonmaterial=1/99” to “Si material/carbon material=30/70” may be satisfied.For example, the relationships of “Si material/carbon material=5/95” to“Si material/carbon material=25/75” may be satisfied. For example, therelationships of “Si material/carbon material=10/90” to “Simaterial/carbon material=20/80” may be satisfied.

Other Components

The negative electrode active material layer 22 may further contain aconductive material in addition to the negative electrode activematerial. The conductive material has electron conductivity. Theconductive material may contain any component. The conductive materialmay contain, for example, at least one selected from the groupconsisting of acetylene black (AB), vapor grown carbon fiber (VGCF),carbon nanotube (CNT), and graphene flake. The blended amount of theconductive material may be, for example, 0.1 parts by mass to 20 partsby mass with respect to 100 parts by mass of the negative electrodeactive material.

The negative electrode active material layer 22 may further contain abinder in addition to the negative electrode active material. The binderbonds solids together. The binder may contain any component. The bindermay contain, for example, at least one selected from the groupconsisting of carboxymethyl cellulose (CMC), styrene butadiene rubber(SBR), polyacrylic acid (PAA), butyl rubber (IIR), polyimide (PI),polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), andvinylidene fluoride-hexafluoropropylene copolymer (PVdF-HFP). Theblended amount of the binder may be, for example, 0.1 parts by mass to10 parts by mass with respect to 100 parts by mass of the negativeelectrode active material.

Positive Electrode

The positive electrode 10 has a sheet shape. The positive electrode 10may include, for example, a positive electrode current collector 11 anda positive electrode active material layer 12. The positive electrodecurrent collector 11 may have a thickness of, for example, 5 μm to 50μm. The positive electrode current collector 11 may contain an aluminum(Al) foil or the like.

The positive electrode active material layer 12 is formed on a surfaceof the positive electrode current collector 11. The positive electrodeactive material layer 12 may be formed on solely one surface of thepositive electrode current collector 11. The positive electrode activematerial layer 12 may be formed on both front and back surfaces of thepositive electrode current collector 11. The positive electrode activematerial layer 12 may have a thickness of, for example, 10 μm to 200 μm.

The positive electrode active material layer 12 contains a positiveelectrode active material. The positive electrode active material layer12 may substantially consist of the positive electrode active material.The positive electrode active material may be a particle group, forexample. The positive electrode active material may have a mediandiameter of, for example, 1 μm to 30 μm.

The positive electrode active material may contain any component. Thepositive electrode active material may contain, for example, at leastone selected from the group consisting of lithium cobalt oxide, lithiumnickel oxide, lithium manganate, nickel cobalt lithium manganate, nickelcobalt lithium aluminate, and lithium iron phosphate.

The positive electrode active material layer 12 may further contain aconductive material in addition to the positive electrode activematerial. The conductive material may contain any component. Theconductive material may contain acetylene black or the like. The blendedamount of the conductive material may be, for example, 0.1 parts by massto 10 parts by mass with respect to 100 parts by mass of the positiveelectrode active material.

The positive electrode active material layer 12 may further contain abinder in addition to the positive electrode active material. The bindermay contain any component. The binder may contain PVdF or the like. Theblended amount of the binder may be, for example, 0.1 parts by mass to10 parts by mass with respect to 100 parts by mass of the positiveelectrode active material.

Electrolyte

The electrolyte is a Li ion conductor. The electrolyte may be a solid, agel, or a liquid. That is, the battery 100 in the present embodiment maybe an all-solid state battery, a polymer battery, or a liquid battery.In the present embodiment, a liquid electrolyte will be described as anexample. The liquid electrolyte may contain, for example, anelectrolytic solution or an ionic liquid.

The electrolytic solution contains a solvent and a supportingelectrolyte. The solvent is aprotic. The solvent may dissolve thesupporting electrolyte. The solvent may contain, for example, at leastone selected from the group consisting of fluoroethylene carbonate(FEC), ethylene carbonate (EC), propylene carbonate (PC), butylenecarbonate (BC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC),and diethyl carbonate (DEC).

The supporting electrolyte contains an ionic compound. The supportingelectrolyte contains Li. The supporting electrolyte may contain, forexample, at least one selected from the group consisting of LiPF₆,LiBF₄, and Li(FSO₂)₂N. The concentration of the supporting electrolytemay be, for example, 0.5 mol/L to 2 mol/L.

The electrolytic solution may further contain various additives inaddition to the solvent and the supporting electrolyte. The additivesmay contain at least one selected from the group consisting of vinylenecarbonate (VC), vinylethylene carbonate (VEC), 1,3-propanesultone (PS),cyclohexylbenzene (CHB), tert-amylbenzene (TAB), and lithium bisoxalateborate (LiBOB), for example.

Separator

The separator 30 is interposed between the positive electrode 10 and thenegative electrode 20. The separator 30 physically separates thepositive electrode 10 and the negative electrode 20 from each other. Forexample, in an all-solid state battery, an electrolyte may function as aseparator.

The separator 30 may have a sheet shape, for example. The separator 30may have a thickness of, for example, 5 μm to 30 μm. The separator 30 isporous. A plurality of pores is formed on the inside of the separator30. The pores can retain the electrolytic solution. The separator 30 mayhave a porosity of 30% to 60%, for example. The porosity can be measuredby mercury intrusion porosimetry.

The separator 30 may be made of polyolefin, for example. The separator30 may be made of polyethylene (PE), for example. The separator 30 maybe made of polypropylene (PP), for example. The separator 30 may have asingle layer structure, for example. The separator 30 may substantiallyconsist of a PE layer, for example. The separator 30 may have amultilayer structure, for example. The separator 30 may be formed, forexample, by laminating a PP layer, a PE layer, and a PP layer in thisorder. A surface of the separator 30 may be coated with a ceramicmaterial, for example. The ceramic material can impart heat resistanceto the surface of the separator 30.

Manufacturing Method of Lithium-Ion Battery

FIG. 6 is a schematic flowchart of a manufacturing method of alithium-ion battery in the present embodiment. The manufacturing methodof a lithium-ion battery in the present embodiment includes <<(A)assembly>> and <<(B) initial charging>>.

(A) Assembly

The manufacturing method of a lithium-ion battery in the presentembodiment includes assembling the battery 100. The battery 100 includesthe positive electrode 10, the negative electrode 20, and theelectrolyte. The details of the battery 100 are as described above. Thebattery 100 is assembled by any method. The negative electrode activematerial contains the precursor of the Si material before the initialcharging. The precursor has a composition represented by the formula(I).

(B) Initial Charging

The manufacturing method of a lithium-ion battery in the presentembodiment includes performing the initial charging on the battery 100.When the initial charging is performed, the precursor (SiO_(x)) reactswith Li. As a result, it is considered that the precursor isdisproportionate in the Si alloy phase and the silicate phase.Furthermore, it is considered that the three-dimensional networkstructure is formed by the growth of the Si alloy phase in thethree-dimensional network. In the present embodiment, the initialcharging is performed such that the three-dimensional network structurehas an average mesh size of 2.8 nm to 3.5 nm.

The initial charging is performed by a charging device. The chargingdevice may be a charging and discharging device. The initial chargingmay be performed in a room temperature environment. For example, theinitial charging may be performed in a thermostat set at 15° C. to 30°C.

In the present embodiment, the initial charging is divided into twosteps. That is, the initial charging includes a first step and a secondstep.

First Step

The first step is charging in the range from the uncharged voltage tothe intermediate voltage. The first step charging may be a constantcurrent (CC) method. In the first step, the charging is performed to anintermediate voltage at a first current rate.

The intermediate voltage is 3.75 V or higher. When the intermediatevoltage is less than 3.75 V, the average mesh size may exceed 3.5 nm.The intermediate voltage may be, for example, 3.75 V to 3.9 V.

The first current rate is lower than 0.5 C. When the first current rateis 0.5 C or higher, the average mesh size may exceed 3.5 nm. The firstcurrent rate may be, for example, 0.1 C to 0.3 C.

Second Step

After the voltage reaches the intermediate voltage, the initial chargingshifts from the first step to the second step. The second step ischarging in the range from the intermediate voltage to the maximumvoltage. The second step charging may be a CC method. In the secondstep, the charging is performed from the intermediate voltage to amaximum voltage at a second current rate.

The second current rate is higher than the first current rate. When thesecond current rate is equal to or less than the first current rate, theaverage mesh size may be less than 2.8 nm. The second current rate maybe, for example, 0.3 C to 1 C.

The maximum voltage is a voltage higher than the intermediate voltage.The maximum voltage is the maximum value of the voltage in the initialcharging. The maximum voltage in the initial charging may be equal tothe maximum voltage in a working voltage range of battery 100. Themaximum voltage may be, for example, 4.1 V to 4.3 V. The maximum voltagemay be, for example, 4.2 V to 4.3 V. The initial charging is completedwhen the voltage of the battery 100 reaches the maximum voltage.

Other Operations

After the initial charging, the battery 100 may be discharged, forexample. After the initial charging, heat aging may be performed on thebattery 100, for example. For example, the battery 100 may be left in atemperature environment of 50° C. to 70° C. for about 24 hours to 48hours.

The lithium-ion battery is manufactured in the manner described above.In the lithium-ion battery (finished product) of the present embodiment,improvement in storage characteristics is expected. It is consideredthat the three-dimensional network structure of the Si alloy phase hasan appropriate denseness.

Hereinafter, examples of the present disclosure (hereinbelow, alsoreferred to as “the present examples”) will be described. Note that, thefollowing description does not limit the scope of the claims.

Manufacture of Lithium-Ion Battery

Various lithium-ion batteries were manufactured by the followingprocedure.

Example 1

(A) Assembly

1. Production of Positive Electrode

The following materials were prepared.

Positive electrode active material: LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ (mediandiameter of 10 μm)

Conductive material: acetylene black

Binder: PVdF

Dispersion medium: N-methyl-2-pyrrolidone

Positive electrode current collector: Al foil

A slurry was prepared by mixing the positive electrode active material,the conductive material, the binder, and the dispersion medium. Thesurfaces (both front and back surfaces) of the positive electrodecurrent collector were coated with the slurry and then dried. In thisway, positive electrode active material layers were formed on thesurfaces of the positive electrode current collector. The composition ofthe positive electrode active material layer was “positive electrodeactive material/conductive material/binder=87/10/3 (mass ratio)”.

A positive electrode raw material was manufactured in the mannerdescribed above. A plurality of positive electrodes was manufactured bycutting the positive electrode raw material.

2. Production of Negative Electrode

A powder of SiO₂ (commercially available product) and a powder ofmetallic Si (commercially available product) were mixed. Thereby, amixed powder was prepared. A reaction vessel was prepared. The reactionvessel had a sealed structure. The reaction vessel was filled with themixed powder prepared in advance. In the reaction vessel, the mixedpowder was heated to a temperature of 1300° C. to 1400° C. under anargon (Ar) atmosphere. As a result, sublimation gas was generated. It isconsidered that the composition of the sublimation gas was SiO_(x)(x=1). By cooling the sublimation gas, a SiO powder was formed. The SiOpowder was collected. The SiO powder was ground.

The following materials were prepared.

Negative electrode active material: precursor of Si material (SiOprepared above), carbon material (commercial artificial graphite)

Binder: “SBR/CMC=1/1 (mass ratio)”

Dispersion medium: water

Negative electrode current collector: Cu foil

The negative electrode active material was prepared by mixing 76 partsby mass of the carbon material and 20 parts by mass of the precursor(SiO). A slurry was prepared by mixing the negative electrode activematerial, the binder, and the dispersion medium. The surfaces (bothfront and back surfaces) of the negative electrode current collectorwere coated with the slurry and then dried. In this way, negativeelectrode active material layers were formed on the surfaces of thenegative electrode current collector. The composition of the negativeelectrode active material layer was “negative electrode activematerial/binder=96/4 (mass ratio)”.

A negative electrode raw material was manufactured in the mannerdescribed above. A plurality of negative electrodes was manufactured bycutting the negative electrode raw material.

3. Formation of Power Storage Element

A separator was prepared. The positive electrode and the negativeelectrode were alternately laminated while the separator was sandwichedbetween the positive electrode and the negative electrode. As a result,a laminate-type power storage element was formed. The power storageelement included seven positive electrodes and eight negativeelectrodes. The positive electrode terminals and the negative electrodeterminals were connected to the power storage element.

4. Liquid Injection

A pouch made of an aluminum laminated film was prepared as an exteriormaterial. The power storage element was accommodated in the exteriormaterial. The electrolytic solution was injected into the exteriormaterial. The electrolytic solution contained the following components.

Solvent: “FEC/EC/DMC/EMC=1/2/4/3 (volume ratio)”

Supporting electrolyte: LiPF₆ (concentration=1.0 mol/L)

After the injection of the electrolytic solution, the exterior materialwas sealed by heat sealing. A test battery was assembled in the mannerdescribed above.

(B) Initial Charging

Two metal plates were prepared. The test battery was sandwiched betweentwo metal plates. The two metal plates were fixed such that apredetermined pressure was applied to the power storage element.

First Step

In an environment with a temperature of 25° C., the battery was chargedto the intermediate voltage of 3.75 V at a first current rate of 0.1 C.The charging was performed by a CC method.

Second Step

In an environment with a temperature of 25° C., the battery was chargedto the maximum voltage of 4.3 V at a second current rate of 1 C. Thecharging was performed by a CC method.

After the initial charging, the test battery was discharged to 2.5 V ata current rate of 0.2 C. The discharge capacity at this time wasconsidered to be an initial capacity. The test battery was manufacturedin the manner described above.

In the present example, two test batteries were manufactured for eachspecification. One of the test batteries was used to measure the averagemesh size. The other test battery was used to evaluate the storagecharacteristics.

Example 2

As shown in the following Table 1, a test battery was manufactured inthe same manner as in Example 1 except that the first current rate inthe initial charging was changed.

Example 3

As shown in the following Table 1, a test battery was manufactured inthe same manner as in Example 1 except that the second current rate inthe initial charging was changed.

Example 4

As shown in the following Table 1, a test battery was manufactured inthe same manner as in Example 1 except that the intermediate voltage inthe initial charging was changed.

Comparative Example 1

As shown in the following Table 1, the initial charging was consistentlyperformed without dividing the initial charging into the first step andthe second step. The current rate was 1 C. A test battery wasmanufactured in the same manner as in Example 1 except for the abovedescription.

Comparative Example 2 and Comparative Example 3

As shown in the following Table 1, a test battery was manufactured inthe same manner as in Example 1 except that the first current rate inthe initial charging was changed.

Comparative Example 4

As shown in the following Table 1, a test battery was manufactured inthe same manner as in Example 1 except that the second current rate inthe initial charging was changed.

Comparative Example 5

As shown in the following Table 1, a test battery was manufactured inthe same manner as in Example 2 except that the second current rate inthe initial charging was changed.

Comparative Example 6

As shown in the following Table 1, a test battery was manufactured inthe same manner as in Example 2 except that the intermediate voltage inthe initial charging was changed.

Evaluation

Average Mesh Size

According to the method described above, the test battery wasdisassembled and the average mesh size was measured in eachspecification. In the present example, EMC was used as the organicsolvent for cleaning the negative electrode.

Storage Characteristics

The test battery was charged to 4.2 V at a current rate of 0.2 C. Thetest battery in a charged state was stored for 28 days in a thermostatset at 60° C.

After 28 days, the test battery was discharged to 2.5 V at a currentrate of 0.2 C in a room temperature environment. Next, the test batterywas charged to 4.1 V at a current rate of 0.2 C. After the charging, thetest battery was discharged to 2.5 V at a current rate of 0.2 C. Thedischarge capacity at this time was considered to be a capacity afterstorage. A capacity retention rate was obtained by dividing the capacityafter storage by the initial capacity. The capacity retention rate isshown in the following Table 1. It is considered that the higher thecapacity retention rate, the better the storage characteristics.

TABLE 1 Storage Initial charging Battery characteristics First stepSecond step Si material 60° C. 28 days First current Intermediate Secondcurrent Maximum Average mesh Capacity rate voltage rate voltage sizeretention rate /C /V /C /V /nm /% Comparative 1 — 1 4.3 8.2 72 Example 1Comparative 0.8 3.75 1 4.3 8 75 Example 2 Comparative 0.5 3.75 1 4.3 7.878 Example 3 Comparative 0.1 3.75 0.1 4.3 1.8 78 Example 4 Comparative0.3 3.75 0.1 4.3 1.7 75 Example 5 Comparative 0.3 3.4 1 4.3 8.5 68Example 6 Example 1 0.1 3.75 1 4.3 3.5 85 Example 2 0.3 3.75 1 4.3 2.883 Example 3 0.1 3.75 0.3 4.3 3.3 84 Example 4 0.1 3.9 1 4.3 3.1 86

Result

As shown in Table 1 above, when the average mesh size is 2.8 nm to 3.5nm, the storage characteristics tend to be improved.

FIG. 4 is a first example of a Si distribution image by STEM-EELS.

FIG. 5 is a second example of a Si distribution image by STEM-EELS. FIG.4 shows a Si distribution image (Si alloy phase) in ComparativeExample 1. FIG. 5 shows a Si distribution image in Example 1. Theobservation magnification of FIG. 4 is the same as the observationmagnification of FIG. 5. It is considered that the three-dimensionalnetwork structure of the Si alloy phase is denser in FIG. 5 (Example 1)than in FIG. 4 (Comparative Example 1).

The present embodiment and the present example are merely examples inall respects. The present embodiment and the present example are notrestrictive. The technical scope defined by the description of claimsincludes all modifications semantically equivalent to the description ofthe claims. The technical scope defined by the description of the claimsincludes all modifications within the scope equivalent to thedescription of the claims.

What is claimed is:
 1. A lithium-ion battery comprising: a positiveelectrode; a negative electrode; and an electrolyte, wherein: thenegative electrode contains a negative electrode active material; thenegative electrode active material contains a silicon material; thesilicon material contains a silicon alloy phase and a silicate phase;the silicon alloy phase has a three-dimensional network structure; thesilicate phase is arranged in a mesh of the three-dimensional networkstructure; and the three-dimensional network structure has an averagemesh size of 2.8 nm to 3.5 nm.
 2. The lithium-ion battery according toclaim 1, wherein the negative electrode active material further containsa carbon material.
 3. A manufacturing method of a lithium-ion batterycomprising: assembling the lithium-ion battery; and performing aninitial charging on the lithium-ion battery, wherein: the lithium-ionbattery includes a positive electrode, a negative electrode, and anelectrolyte; the negative electrode contains a negative electrode activematerial; the negative electrode active material contains a precursor ofa silicon material; the precursor has a composition represented bySiO_(x) where a relationship of 0<x<2 is satisfied; the initial chargingincludes a first step and a second step; in the first step, the chargingis performed to an intermediate voltage at a first current rate; in thesecond step, the charging is performed from the intermediate voltage toa maximum voltage at a second current rate; the first current rate islower than 0.5 C; the second current rate is higher than the firstcurrent rate; and the intermediate voltage is 3.75 V or higher.
 4. Themanufacturing method of the lithium-ion battery according to claim 3,wherein the negative electrode active material further contains a carbonmaterial.